bought blacklisted ATT S6 active..but it is still working?? how is it working?

Hello,

So, I just bought a BNIB S6 active...ATT says it is blacklisted. However, I just put my ATT SIM in it and it is working fine, just like its not blacklisted..the data works, SMS works, etc...

all of the "imei checker" sites also say that it is blacklisted also

so, my question is...why do the sites AND ATT say its blacklisted, but still working just fine? Is it something in the system that is laggy, and in a few days it will eventually block the IMEI and my SIM card??